Renewable resources are great, but they bring a new element of uncertainty to a power grid. This element can lead to failure in surprising ways, according to a new paper.

A team of researchers built a model of power grids that transport electricity from solar and wind power. That means that there are places where the grid receives fluctuating inputs of power, since levels of sunlight and wind and vary.

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You might think that a power grid failure would be the result of a single windstorm or thunderstorm at one node. But that’s not the case, based on the researchers’ model, which uses the tools of statistical physics, a field whose math describes lots of interacting and randomly moving pieces. They found that small, unusual fluctuations combined over the entire network were more likely to take the power lines down, according to the paper published in Physical Review Letters.

The model was also able to predict where a line would fail and how the failure would propagate, given information about the grid and the weather. The researchers compared their model to actual data from the German transmission grid, which gets about 33% of its power from renewable sources, and things seemed to line up.

Being a model, there are some assumptions built in that might not correspond to the real world. But perhaps these are the types of things that must be taken into account when designing a power grid that relies in part on the ever-temperamental weather.